@charliekthrn

Hey

There are a few things more you should consider before going down this path.   Firstly how fast is your internet connection?    and  what Router / ISP Device do you have currently?    If you use something like WiFi Analyser for Android, what is the -db signal you are getting at the PC? 

imho a repeater should be the last thing you consider, especially if you are going to be gaming on it.   Repeaters massively increase latency (ping) and as its a second device in the middle, the speed of transfer is halfed.

Lets assume you have 300mbps internet using WiFi5 (AC), this wifi would be solid at around 450mbps in most setups, add a repeater and its now 220ish.. so slower than your internet speed, with twice the latency.

Mesh systems like Deco.. while not perfect are a LOT better as they talk to eachother so latency, while increased is better and throughput can be good if devices are of triband.   Repeaters just repeat, mesh is a different beast.

Powerline adaptors, are reliable enough but SLOW..  again depends on your speed.

To answer some of your questions

1. Obstacles: There are a few walls and floors between my router and the gaming PC. Could this be a major factor?  YES.   each floor / wall will take -3db from the signal, while this seems small... its not.    db works in a logarithmic pattern, so -3db is a 50% loss in signal.   Therefore 2x walls and you have lost 75% of your signal.  Any repeater of Mesh should be in the middle of these to re-amplify the signal

2. Interference: I've heard nearby devices can mess with the Wi-Fi signal. Any tips on identifying and minimizing interference?   Something like WiFi Analyser for Android will tell you this (also a windows version).  Basically as a rule of thumb, any neighbour on your channel with a signal less than -70db will be a source of interference.  If its sitting there at -75 for example, its so weak its not an issue.   -50db  you have an issue.   You will need to consider channels when setting this up.

3. Router Placement: I've checked the placement of my router, but maybe there's something I'm missing. What's the optimal setup for the best coverage?  In the centre of the house, antenna pointed vertically.   Not always possible however.   It also depends on your router and what antenna it has, if you can provide the model that would help

4. Range Extenders: Are TP-Link range extenders good for gaming setups? Do they significantly improve speed, or are there downsides I should be aware of?   See Above..

5. Alternatives: Are there other solutions, like powerline adapters or mesh Wi-Fi systems, that might work better?   Give us more info on your setup first, however unless budget is a real concern for you.  I would choose MESH over Repeater all day long.   Powerline has its uses too, but speed is a problem there.